1. Field of the Invention
The present invention relates to a high-pressure discharge lamp for a projector or the like, and a light source device using the high-pressure discharge lamp.
2. Description of the Related Art
A high-pressure discharge lamp for a light source of a projector or the like includes an arc tube portion encapsulating mercury vapor within an internal space thereof, and a pair of seal portions extending from both ends of the arc tube portion for sealing the internal space of the arc tube portion. The amount of light emitted from the high-pressure discharge lamp increases as the pressure of mercury vapor encapsulated in the arc tube portion increases. To emit a larger amount of light, it is required to raise the pressure of mercury vapor encapsulated in the arc tube portion. Therefore, an enhanced pressure tight performance is required for the seal portions.
In order to meet this requirement, the pressure tight performance of the seal portions has been enhanced as in a high-pressure discharge lamp 1 shown in FIG. 10, wherein metal foils 2 are embedded with respective pre-seal glasses 3; the pre-seal glasses 3 are embedded in respective seal portions 5 of a envelope 4 thereby enhancing the pressure tight performance of the seal portions 5 (see patent document 1: Japanese Patent Publication No. 2004-241375).
In the high-pressure discharge lamp 1, dielectric breakdown occurs between electrodes 6 by applying a high starting voltage between the electrodes 6. Mercury which is encapsulated in the arc tube portion 7 is excited by the arc which is generated by the dielectric breakdown and produces to emit light. The high-pressure discharge lamp 1 is difficult to start because such a high starting voltage is needed to be applied.
To start the high-pressure discharge lamp 1 easily with a low starting voltage, a space 8 is formed between the envelope 4 and the pre-seal glass 3 to encapsulate inert gas therein and a conductor 9 is placed to extend around the outer periphery of one of the seal portions 5 surrounding the space 8. To generate electric discharge between the metal foil 2 within the seal portion 5 and the conductor 9 for emitting ultraviolet rays, a high voltage is applied between the conductor 9 and the metal foil 2. The surface of the electrode 6 which is irradiated with the ultraviolet rays emits electrons to generate dielectric breakdown between the electrodes 6 easily, whereby the starting voltage for the high-pressure discharge lamp 1 can be lowered.
In the high-pressure discharge lamp 1 described in patent document 1, a material having a lower softening point than a material of the envelope 4 is forced to be used for the pre-seal glasses 3. Specifically, each of the metal foils 2 expands by heat used to heat-seal the envelope 4 and each pre-seal glass 3. Since the metal foil embedding portion of each pre-seal glass 3 wrapping the metal foil 2 is heat-sealed with the internal surface of the seal portion 5, a stress produced with expansion of the metal foil 2 reaches to the heat-sealed portion between the pre-seal glass 3 and the seal portion 5. And the stress causes cracking which might result in leakage of mercury vapor encapsulated within the internal space of the arc tube portion 7. To obviate this problem, the high-pressure discharge lamp 1 described in patent document 1 imparts the seal portions 5 with an enhanced pressure tight performance by employing the material having a lower softening point than the material of the envelope 4 for the pre-seal glasses 3 and embedding each pre-seal glass 3 in respective the seal portions 5 of the envelope 4 under compression stress. The compression stress exerted on the pre-seal glass 3 acts to suppress the stress produced with expansion of the metal foil 2 thereby preventing from cracking.
However, use of different materials for the envelope 4 and the pre-seal glass 3 results in disadvantages including: an increase in the cost of manufacturing the high-pressure discharge lamp 1; an increase in the man-hour required for the manufacture of the high-pressure discharge lamp 1; and a decrease in the stability of raw glass material supply. In addition, the envelope 4 and the pre-seal glass 3 are formed from different materials so that thermal expansion coefficients are naturally different from each other. For this reason, the difference in thermal expansion coefficients might cause cracking at the heat-sealed interface between the envelope 4 and the pre-seal glass 3 as the high-pressure discharge lamp 1 is turned on and off repeatedly.
Though the above-described technique makes it possible to start lighting the lamp at a starting voltage that is lowered to a certain degree, a high-pressure discharge lamp which can be started at a lower starting voltage has been required.